1. for ... in 語句

for (let variable in object)  { ... }

https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Statements/for...in

以任意次序迭代一個物件的可列舉屬性。

var obj = {a: 1, b: 2};
for (let i in obj) {
    console("key:" + i + ", value:" + obj[i]);
}

陣列Array的遍歷，i則為數字

2. 迭代器遍歷

https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Iterator

如Array

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach

arr.forEach((value, index) => {
     console.log(value);
});

https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Statements/for...of

for...of語句在可迭代物件(包括 Array, Map, Set, String, TypedArray，arguments 物件等等)上建立一個迭代迴圈，對每個不同屬性的屬性值,呼叫一個自定義的有執行語句的迭代掛鉤.

但對於普通物件是不適應的。原因是沒有實現參考下文。

https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Guide/iterable